1. Field of the Invention
The present invention relates to a terminal apparatus for transmitting/receiving data on a communication network. More particularly, the present invention relates to a terminal apparatus for transmitting/receiving data on a communication network having a plurality of communication systems different from one another (hereinafter also simply referred to as “different communication systems”).
2. Description of the Background Art
Conventionally, wireless LAN systems and power line communication systems have been commercialized as communication systems in which data is transmitted in packets among a plurality of terminals. For wireless LAN systems, there are IEEE802.11, IEEE802.11b and IEEE802.11g standards which utilize a 2.4-GHz band. These wireless LAN systems adopt different modulation techniques to improve communication speed every time the specification is extended. Further, the wireless LAN systems are standardized in a manner which enables the different standard systems to coexist and be connected to one another.
In IEEE802.11, an autonomous distributed access control is achieved by performing an access control called DCF (Distributed Coordination Function) using a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) scheme. In the CSMA/CA scheme, a terminal which tries to transmit a signal checks (carrier sense) the state of use of a wireless channel before transmission of the signal in order to avoid collision with a signal transmitted by another terminal. When the wireless channel is not used (idle), a signal is immediately transmitted. When the wireless channel is used (busy), transmission is postponed until the wireless channel is idle.
To determine whether or not the wireless channel is idle, IEEE802.11 defines an IFS (Inter Frame Space). When no signal is detected over the defined IFS time or more, a terminal which tries to transmit a signal determines that the wireless channel is idle. The IFS is defined to have a fixed length. A plurality of lengths of IFS may be defined. In this case, the priorities of terminals or frames are controlled by assigning each terminal an IFS having a length corresponding to the priority thereof.
FIG. 36 is a diagram for explaining a control of the priority using IFS. IFSs (SIFS, PIFS, DIFS) having different lengths are assigned to frames, depending on the priority. The priorities of the frames are increased in order of SIFS, PIFS and DIFS. For example, it takes a short time for a terminal which wants to transmit a frame assigned with the short IFS to determine whether or not the wireless channel is idle. Therefore, a frame assigned with the short IFS is transmitted earlier than a frame assigned with a low priority. Therefore, a frame having a high priority has more opportunities of transmission than a frame having a low priority.
When the wireless channel is busy, a terminal waits until the wireless channel is idle. When the wireless channel is idle, a backoff algorithm is executed so as to avoid subsequent collision. The backoff algorithm is used to avoid the collision of packets which is caused by simultaneous transmission by a plurality of terminals. The probability of collision is highest at a time immediately after a certain terminal completes transmission and a wireless channel is then transitioned from the busy state to the idle state. This is because terminals ready for transmission which recognizes that the wireless channel comes to be the idle state start transmitting a signal simultaneously.
FIG. 37 is a diagram for explaining an access control method using a conventional CSMA/CA scheme. Hereinafter, an access control method using a CSMA/CA scheme, to which the backoff algorithm is applied, will be described with reference to FIG. 37.
An access point AP and stations STA1, STA2 and STA3 always perform carrier sense and monitor the state of use of a wireless channel. Here, it is assumed that DIFS (Distributed IFS) is used as IFS. When the wireless channel is in the busy state, each station generates a random number within the range of zero to CW (Contention Window: a range in which random numbers from zero to uniform distribution are generated in the backoff algorithm). Each station determines a backoff time based on the random number thus generated. Thereafter, each station decreases the backoff time during a time when the wireless channel is idle. At a time when the decreased backoff time becomes zero, packet transmission is started. In FIG. 37, the backoff time of the access point AP is the first to become zero, and therefore, the access point AP transmits a packet. When another terminal starts transmission before the remaining time becomes zero, the other stations (in FIG. 37, the stations STA1, STA2 and STA3) become ready for transmission again. The remaining backoff time is decreased again from a time when the wireless channel becomes idle. In FIG. 37, by decreasing the remaining backoff time again, the backoff time of the station STA2 is the first to become zero, and therefore, the station STA2 transmits a packet. Note that a time until the wireless channel becomes idle is stored as an NAV (Network Allocation Vector) value in a header of a transmission packet. A terminal ready for transmission can know the next idle time by waiting for a time matching the NAV value stored in the transmitted packet. The NAV value varies depending on the length of data constituting a packet or a modulation rate used in transmission.
The backoff algorithm of IEEE802.11 is called a “binary exponential backoff algorithm”. The range CW of a random number generated is given by parameters, i.e., a minimum value CWmin and a maximum value CWmax. In the initial random number generation, the value of CW is set to be CWmin. The CW value is doubled as the number of retransmissions is increased by one. After the CW value reaches CWmax, CW becomes a constant value. Thus, the CW value is increased with an increase in the number of retransmissions. In other words, as traffic increases and becomes denser, the number of variations of backoff time is increased. Therefore, data collision is more likely avoided.
When normally receiving a packet, a station returns ACK (Acknowledgement) to the sender after the SIFS (Short IFS) interval from the time of completion of the reception. SIFS is the shortest IFS. Therefore, the ACK signal is transmitted with the highest priority. After transmitting a packet, if an ACK signal is not returned within a predetermined time from the time of completion of data transmission, a terminal determines that the transmission has failed and retransmits the packet.
A physical layer of IEEE802.11 is divided into a PMD (Physical Medium Dependent) sub-layer and a PLCP (Physical Layer Convergence Protocol) sub-layer. The PLCP sub-layer is positioned between the PMD sub-layer and an MAC layer, and absorbs the difference among three modulation/demodulation schemes defined in the PMD sub-layer to unify the interface between the MAC layer and the physical layer. The IEEE802.11b standard is the conventional IEEE802.11 standard with an extended DS-SS scheme and is downwardly compatible with IEEE802.11. In the IEEE802.11b standard, 5.5 Mbps and 11 Mbps are defined as transmission rates in addition to 1M bps and 2M bps of the DS-SS scheme. The DS-SS scheme uses an 11-bit Barker code for spread spectrum modulation. In the IEEE802.11b standard, the transmission rate is increased by adding the CCK scheme which uses a complementary code. IEEE802.11b defines four transmission rates. In IEEE802.11b, the transmission rate of a preamble portion and a header portion is fixed to 1 Mbps. The header region stores information about a transmission rate used for a data portion. Thereby, data can be correctly demodulated by a receiver.
FIG. 38 is a diagram showing a packet structure of the conventional IEEE802.11b. In FIG. 38, “SYNC” is a field for indicating a synchronization signal and “SFD” is a field for indicating the start of a frame and the head of a physical layer, which is an abbreviation of “Start Frame Delimiter”. These fields constitute a PLCP preamble portion.
“SIGNAL” is a field for indicating the transmission rate of the data portion. “SERVICE” is a field (CCK, PBCC) for identifying a high-speed modulation scheme. “LENGTH” is a field for indicating a time required to transmit data (unit: mS). “CRC” is a field for indicating a cyclic redundancy check code. These fields constitute a PLCP header portion.
The PLCP preamble portion and the PLCP header portion constitute a long preamble.
“MPDU” is a MAC protocol data unit and is a field for storing data. The data is modulated by any of 1-Mbps DBPSK (spread with a Barker code), 2-Mbps DBPSK (spread with a Barker code), and 5.5- or 11-Mbps CCK modulation.
“PPDU” is a PHY protocol data unit.
FIG. 39 is a diagram showing packet structures of IEEE802.11, 802.11b and 802.11g. The heads of the packet structures each have a 1-Mbps preamble which is DS-modulated. Since the preamble is DS-modulated, a terminal can recognize it no matter which standard is employed in the terminal. Data is subjected to DS modulation, CCK modulation or OFDM modulation, depending on a standard to which a destination terminal belongs. Thereby, the coexistence and compatibility of these standards are held.
As a power line communication system, HomePlug 1.0 has been developed and commercialized by HomePlug Alliance, US (see Yu-Ju Lin et al., A Comparative Performance Study of Wireless and Power Line Networks, IEEE Communication Magazine, April 2003, p54–p63). The power line communication system achieves 14-Mbps communication using a power line in a room.
FIG. 40 is a diagram showing a frame structure and a protocol of HomePlug 1.0. In FIG. 40, as with “DIFS” of IEEE802.11, “CIFS” is an inter-frame spacing. “Priority Resolution0” and “Priority Resolution1” are priority times. A terminal which transmits a symbol for a priority time has a higher priority than that of a terminal which does not transmit a symbol during the priority time. The two priority times can be used to indicate four priorities. Terminals having the highest priority can be transitioned to the subsequent “Contention” phase. In the “Contention” phase, an algorithm similar to the backoff algorithm of IEEE802.11 is executed to avoid collision. A terminal which has started earliest in the “Contention” period transmits the subsequent data packet. “Data” has “preamble” and “frame control” at a head thereof. These have functions similar to those of the PLCP preamble and PLCP header of IEEE802.11. “Frame body” has a variable length which varies between 313.5 μSec to 1489.5 μSec, depending on the data length and the modulation rate. A busy time substantially corresponding to the length of the “frame body” is stored in NAV information within the “frame control”. A terminal which has received the “frame control” waits for the busy time indicated by the NAV before starting measuring CIFS for the next transmission.
As described above, the conventional IEEE802.11, IEEE802.11b and IEEE802.11g use the DS-SS scheme common to the preamble portion and the header portion, and apply a high-efficiency modulation scheme to data, thereby holding the compatibility and the coexistence. However, in this method, the growth rate of the throughput of data transmission in an upper layer is made very small in spite of the increased speed of the physical layer. This is because, as the speed of the physical layer is increased, a time occupied by the preamble portion and the header portion is relatively elongated.
FIG. 41 is a diagram for explaining that the time occupied by the preamble portion and the header portion is relatively elongated. As shown in FIG. 41, the preamble is DS-modulated in all of IEEE802.11, IEEE802.11b and IEEE802.11g, and therefore, a time required to transmit the preamble is the same. On the other hand, data is DS-modulated in IEEE802.11, CCK-modulated in IEEE802.11b, and OFDM-modulated in IEEE802.1g. Therefore, as the standard is extended, a time occupied by the preamble portion and the header portion is relatively elongated.
According to a quantitative calculation, in IEEE802.11b, the PHY speed is 11 Mbps, while the throughput of an upper layer is about 5 Mbps. However, even if the PHY speed is increased by a factor of about 5 to 54 Mbps, the throughput of the upper layer is increased by a factor of 2 to about 10 Mbps (Tetsu Sakata (NTT Access Network Service Systems Laboratories), Interface, P. 43, FIG. 32, Feb. 1, 2003, CQ Publishing). Particularly in VoIP (Voice Over IP: voice communication utilized in IP telephone or the like), a small amount of data (32 kbps, etc.) is regularly transmitted, so that the proportion of the header in each packet is further increased, resulting in very poor efficiency.
In the high-speed communication system of IEEE802.11b or later, DS-SS scheme packets of IEEE802.11 need to be received. In order to coexist with a conventional scheme, a conventional scheme transmission/reception circuit needs to be mounted. Therefore, the circuit area of LSI is increased, leading to an increase in cost. Particularly, the reception circuit needs to comprise components, such as an AGC circuit, an AD conversion circuit, a demodulation circuit, an error correction circuit and the like, which have a large influence on cost.